I got an email or three yesterday talking about how the new application I'd written wasn't sending out notification emails properly. Since I'd tested it out numerous times to ensure email wasn't going to be a problem from my end, I was a bit surprised. I figured it had something to do with bad data e.g. bad email addresses etc that was causing the problem.
After an email that I sent to my gmail account didn't show up even after 5 minutes, I decided to look at my mail server to see if it had recorded anything. I pulled up the admin console for my newly installed instance of Zimbra and saw about 400 deferred emails on the server status page. I was quite surprised that all those people hadn't complained. Regardless, I selected all the emails and re-queued them for delivery. Each and every one of them failed again. Errors ranged from connectivity errors to my ISP's mail relay (smtp.comcast.net) to other weird errors once my mail server connected to the relay host. Finally I tried my tried and true method - telnetting to port 25 on smtp.comcast.net and manually going through a mail session from helo to the final period. I couldn't even connect to port 25 on the mail relay server!
It turns out Comcast had recently removed access to port 25 on its mail relay server where "recently" refers to November 28th. I'm not entirely certain why I didn't see any issues even at the end of December and beginning of January when I launched the application. In any case, not only had they switched to using port 587 instead of 25, they were also requiring authentication using the username/password associated with your Comcast account to even send email via this relay.
Fair enough. Now to configure Zimbra to support an authenticated mail relay. Luckily I got help from the Zimbra Wiki. Following the instructions to the letter (except for substituting hostnames, usernames and passwords appropriately) I was able to get mail working again.
Now I just have to wait for 400 emails to get delivered 3 at a time - or whatever the absurdly low limit is for the Comcast mail relay.